The present invention relates generally to the field of fiber optic cables. In particular, the present invention is directed to a novel construction of buffer tubes for fiber optic cables which offers a way to access the optical fibers inside a buffer tube while reducing the risk of damaging the fibers, as sometimes occurs when cutting the outer layer of the buffer tube.
Optical fiber cables have been used for many years to transmit information at high rates and very long distances. The transmission medium of the optical fiber cable are delicate optical fibers. Ordinarily, these optical fibers reside in a buffer tube or in a core tube. Hereinafter, for the sake of simplicity, buffer tubes shall also include core tubes. The buffer tube protects the optical fibers residing therein. The buffer tube may be fabricated from a single material or from a combination of two or more materials.
In many fiber optic cable designs, it is desirable to have a buffer tube made from a material with a high Young""s modulus. The use of a material with a high Young""s modulus results in a cable with a buffer or core tube having relatively high tensile and compressive resistance capability, a trait useful in protecting the optical fibers in the event the cable is twisted, stretched or compressed. Also, it is important to select a material for the buffer tube that has a low thermal expansion coefficient. Too much shrinkage or expansion of the buffer tube caused by temperature changes could place tensile or compressive loads on the optical fibers residing therein. High tensile or compressive loads can result in damaged or broken optical fibers.
Known buffer tube designs, either single- or multi-layer, have an additional disadvantage in the difficulty of accessing the optical fibers mid-span. This problem is exacerbated by the recent tendency in optical fiber design to pack greater numbers of optical fibers into smaller buffer tubes. As the fiber density inside tubes increases, free space inside the tubes decreases, and it is more likely to damage optical fibers when a buffer tube is opened with a cutting tool.
Cutting tools for mid-span access of buffer tubes are typically designed so that the blades cut just through the buffer tube wall and extend only a limited distance into the tube cavity. The purpose of making a tool with a carefully controlled depth of cut is to achieve access into the buffer tube with minimum fiber damage. In fact, some recent tools are designed so that the blades cut only partially through the tube wall. The tube is then stressed by hand or with a tool such that the remaining portion of the wall is broken. However, this procedure may be difficult, because buffer tube materials typically do not break readily in order to meet the design constraints discussed above.
Therefore, an improved buffer tube is sought that allows for efficient access to mid-span inner optical fibers without damage to the fiber, and yet satisfies the structural constraints necessary for buffer tube walls.
The present invention is designed to overcome the limitations that are attendant upon the use of the prior art buffer tubes and mid-span fiber accessing techniques. The present invention therefore offers a buffer tube design which has improved access to mid-span fibers, high strength, and low cost. This is achieved by creating a two-layer buffer tube with an inner layer that is notch-sensitive, and an outer layer that has much lower notch-sensitivity and higher strength relative to the inner layer.
Notch-sensitivity, as will be understood by one skilled in the art, is a property of a material that relates to the ease with which a cut, or notch, when made in the material, continues to propagate through the material. When a notch is cut in a highly notch sensitive material, it will propagate through the material with little or no physical intervention by the user.
A negative relationship exists between notch sensitivity and the exertion required to propagate a cut. A material having a gradient of notch sensitivity, whereby notch sensitivity increases towards the center, will require less exertion to propagate a cut towards the center relative to the outside of the material.
According to another aspect of the invention, the buffer tube wall may comprise a single layer with notch-sensitivity increasing towards the inside surface of the buffer tube wall. One embodiment of this invention includes a cellular, or foamed, polymer material with higher cell density towards the inner surface of the buffer tube wall.
A cutting tool containing blades is used to cut the outer (less notch-sensitive) portion of the buffer tube. The inner portion of the buffer tube is then split open with little effort due to the high notch-sensitivity of the inner portion of the buffer tube. Thus, it is possible to greatly reduce the risk of damaging the optical fibers when accessing mid-span optical fibers.
The invention will be more fully understood when reference is made to the following detailed description taken in conjunction with the accompanying drawings.